How to achieve uniform Inset and Extrude on multiple nonplanar surfaces?
Something along these lines might do it.. just following my nose, someone might find a snappier way..

The inset of the outer edge is not uniform

I think the only reason for this is that you scaled your cube in Object Mode, and it's carrying an object-level non-uniform scale. CtrlA &gt; Apply the scale of your object, and if your inset is set to even, it should be even.

I have no idea how to loop cut the vertical slot

It's not a loop-cut, because it's not a loop. K Knife, with C to constrain its direction orthogonal to your view, should do that. Cut from an orthographic front view. The only problem is, how to get the cut right in the middle..


I EZ extruded the top edge up in Z..

CtrlX Dissolved the odd vertices, and Edge menu &gt; Subdivided the top edge once to give me a guide-point in the middle. Then, looking from the front, You can K with C to constrain, cut an edge straight down the middle of the front region. You can discard the guide-edge once you're done with it.

Then, as you have before, you can I inset the whole front region twice, remembering or CtrlC copying the second distance, the width of your rebate. Now select the central cut line and CtrlB Bevel it by half that distance. (You can paste in the copied Insert distance and add a /2 after it into the width field of the Bevel's adjust-last-operation panel)

Then E extrude as before, for even, centered rebates:




You can SY (or whatever dimension) 0 with the Pivot set to Median, to flatten the backs of your grooves.